In any centrifuge containing a density gradient, the gradient material tends to diffuse from an area of high concentration to an area of low concentration even against high centrifugal forces.
In a flow centrifuge the material flowing across the gradient front enters the rotor at the center and travels to the front of the gradient on one radial path, the inflow channel, and returns on another radial path, the outflow channel, leaving behind in the gradient the particles desired to be separated. The high centrifugal forces acting on the material as it is traveling radially are exactly balanced so that very little pressure is required to flow liquid through the rotor at speed.
Despite forward diffusion of the gradient material against the gravitational field, the gradient material does not enter the inflow radial channel because there is a constant inflow of liquid pushing the gradient material back into the rotor. In the case of the outflow radial channel, however, the gradient material can diffuse into this channel with the result that the density of the liquid in the outflow radial channel becomes greater than that of the inflow radial channel. The resulting imbalance in the density of the material in the inflow radial channel versus the outflow radial channel renders flow of material impossible (gradient lock).